Automobile head-up display system with apparatus for positioning source information

ABSTRACT

An optical display system (10) presents visual source information to an observer (18) The display system includes a vision unit (14) that has reflective surfaces (16) through which the observer can view an outside world scene and which reflect source information emanating from an information source (22) for display to the observer In a preferred embodiment, the optical display system constitutes a head-up display system for an automobile and the observer is the driver of the automobile. The vision unit constitutes an automobile windshield with or without a reflection enhancement material and whose inner and outer surfaces reflect source information carried by light propagating form the information source, such as a liquid crystal display (32). A projection lens system (24) positioned between the inner surface of the windshield and the information source has optical light-directing properties for compensating for optical aberrations introduced by the nonplanar windshield surface. The projection lens system includes an aspheric element (262) that is unique to a specific aspheric windshield shape and remaining elements that are common to a great number of different windshield shapes. A positioning mechanism ( 130) allows the driver to adjust the vertical position of the source information (60) reflected by the windshield within a total display field of view for optimal viewing by a seated driver. The positioning mechanism also automatically changes the distance between the display image and the driver as a function of vehicle speed to promote safe vehicle operation.

This is a divisional of application Ser. No. 07/409,954, filed Sep. 15,1989, now U.S. Pat. No. 4,961,625, which was a continuation ofapplication Ser. No. 07/098,528, filed Sep. 18, 1987, now abandoned.

TECHNICAL FIELD

The present invention relates to optical display systems and, inparticular, to a head-up display system for a motor vehicle in which thewindshield reflects light carrying visual vehicle data and statusinformation for display to an occupant of the vehicle as the occupantviews the outside world scene through the windshield.

BACKGROUND OF THE INVENTION

A head-up display system is an optical display system that employs acombiner element through which an observer views an outside world sceneand which reflects visual source information for display to theobserver. The source information is typically carried by light emanatingfrom the display surface of a cathode-ray tube. Head-up display systemsare typically installed in fighter aircraft so that the pilot cansimultaneously monitor critical flight information and observe eventsoccurring outside the aircraft. Conventional combiner elements are ofspherical shape and are designed to cooperate with a relay lens tocollimate the source information-carrying light so that the displayinformation appears to be coming from optical infinity.

The use of head-up display systems in motor vehicles would likelyincrease the safe use of and solve space availability problems in suchvehicles. In particular, the dashboard area of an automobile has becomesusceptible to clutter as the trend toward the inclusion of moreinstrumentation and convenience features continues. The problem ofdashboard clutter has heretofore been remedied by the placement ofessential control knobs and switches on the steering column. The use ofa head-up display system would permit projection of certain vehiclestatus and performance information, such as engine RPM, vehicle speed,and turn signal indications, forward of the vehicle for display to thedriver after reflection from a reflection enhancement material appliedto or embedded in the windshield. Projecting such information out infront of the vehicle minimizes the time the driver needs to view theinformation, thereby enhancing safety in vehicle operation.

A problem inherent in displaying the source information to the observerin this manner is that reflecting light off of a nonplanar surfaceoptically aberrates the light carrying the display information. Thesurfaces of windshields designed for contemporary automobiles are ofcomplex shapes in that they are typically aspheric with differentcurvatures in different sections lying in mutually orthogonal planes.The complex shapes stem from aesthetics and from requirements thatautomobiles exhibit better aerodynamic performance. These requirementsdictate that the windshield be inclined at a relatively large slope,i.e., at a "low angle" and be curved near the hood and roof lines of thevehicle so that the windshield forms a complex curvature between its topand bottom margins. The complex shape of the windshield necessitates,therefore, the use of a correction lens having optical light directingproperties that compensate for those of the windshield that wouldotherwise aberrate light carrying the display information.

A problem arising from the need for a correction lens in an automobilehead-up display system is that different automobile models or bodystyles generally require windshields of different profiles. Correctionlenses characterized by different optical light directing propertieswould, therefore, be required to compensate for the optical aberrationsintroduced by windshield of different profiles.

The windshield must have adequate light transmission properties for theobserver to view the outside world scene. Reflection enhancement devicesfor windshields can be of the wavelength nonselective type, such as athin metallic or dielectric coating, or of the wavelength selectivetype, such as a multilayered dielectric coating or a hologram applied toa surface of the windshield or embedded within the laminae of thewindshield. A process for embedding a hologram between two laminae of awindshield is described in U.S. Pat. No. 4,842,389 for "Vehicle DisplaySystem Using a Holographic Windshield Prepared to Withstand LaminationProcess."

The use of a windshield reflection enhancement device imposes certainsystem design constraints, one of which being the reduction of thephotopic transmission of light (i.e., the transmission of light carryingthe outside world scene) to the observer. The photopic transmission canbe increased by using a wavelength selective device, which reflectslight over a narrow bandwidth and thereby maximizes the windshieldphotopic transmission. The use of a wavelength nonselective deviceentails a direct tradeoff between display brightness and photopictransmission; whereas, the use of a wavelength selective device offersmaximum display brightness and photopic transmission. Another constraintis that the windshield reflection enhancement device be of sufficientquality so that it remains in place and wrinkle free.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide a head-updisplay system for an automobile or other vehicle.

Another object of this invention is to provide such a head-up displaysystem that presents substantially aberration-free source information toan occupant in a vehicle having a windshield of complex shape.

A further object of this invention is to provide in a head-up displaysystem a correction lens that is easily adaptable to correct foraberrations introduced by windshields of different shapes.

Still another object of the present invention is to provide a head-updisplay system using a windshield that employs a reflective surface ofhigh quality and provides maximum transmission of light carrying theoutside world scene to an observer situated in the vehicle.

Yet another object of the present invention is to provide a head-updisplay system having adequate display brightness.

The present invention is an optical display system for presenting visualsource information to an observer. The display system includes a visionunit that has a reflective surface through which the observer can viewan outside world scene and which reflects source information emanatingfrom an information source for display to the observer. In a preferredembodiment, the optical display system constitutes a head-up displaysystem for a motor vehicle such as an automobile. The vision unitconstitutes an automobile windshield with or without a reflectionenhancement material and whose inner and outer surfaces reflectinformation source carried by light propagating from the informationsource, such as a liquid crystal display. The inner and outer surfacesof the windshield are of aspheric shape having different curvatures indifferent sections lying in mutually orthogonal planes. The asphericwindshield profile introduces aberrations into the light carrying thesource information as it is reflected toward the observer.

A projection lens system positioned between the inner surface of thewindshield and the information source has optical light-directingproperties for compensating for these optical aberrations. To facilitatecompensation of optical aberrations introduced by windshields ofdifferent shapes, the projection lens system includes an asphericelement that is unique to a specific windshield shape and remainingelements that are common to a great number of different windshieldshapes.

The projection lens system directs the light carrying the sourceinformation for reflection off the inner and outer surfaces of thewindshield in a region that is substantially outside of the observer'sprimary operational field of view of the outside world scene. Thedimensions of the display surface of the information source are scaledso that the source information displayed is contained within theobserver's instantaneous field of view from different head positions.

The display surface of the information source is operatively connectedto a positioning mechanism that moves the display surface in thedirections transverse to and collinear with the optic axis of thehead-up display system. The positioning mechanism allows the observer toadjust the vertical position of the source information reflected by thewindshield within the total display field of view defined by theprojection lens system for optical viewing by a seated driver. Thepositioning mechanism also automatically changes the distance betweenthe display image and the driver as a function of vehicle speed topromote safe vehicle operation.

The information source preferably comprises a transmissive liquidcrystal display that is illuminated by a high intensity broad bandmultispectral light source. Thin film color transmissive filters on theliquid crystal source produce multicolored images that are reflected bythe windshield. It has been determined that a windshield having awavelength selective coating that enhances reflectance of a certaincolor of light can also reflect other colors of light by means of thereflection properties of the external windshield surfaces themselves.Such a wavelength selective element can, if desired, be used to equalizethe reflectance of different colors of light carrying the sourceinformation.

Additional objects and advantages of the present invention will beapparent from the following detailed description of a preferredembodiment thereof, which proceeds with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an optical display system of the presentinvention installed as a head-up display in an automobile.

FIG. 2 is a plan view of the automobile windshield of FIG. 1 showing theobserver's operational field of view of the outside world scene and theobserver's display field of view of the source information.

FIGS. 3A and 3B are fragmentary cross sectional views taken along lines3--3 of FIG. 2 showing the source information light reflecting surfacesof, respectively, reflection nonenhanced and reflection enhancedautomobile windshields.

FIG. 4 is a graph showing the light reflection characteristics of asingle glass windshield surface as a function of different angles ofincident light.

FIG. 5 is a plan view with portions cut away to show in the automobileof FIG. 1 the projection lens system and the information source mountedimmediately below and forward of the upper surface of the dashboard.

FIGS. 6 and 6A show from the observer's head centered position thealignment of the instantaneous display field of view within the totaldisplay field of view of the head-up display system of FIG. 1.

FIGS. 7A-7D show the instantaneous field of view of the head-up displaysystem of FIG. 1 for, respectively, observer head positions 1.25 inchesto the left, 1.0 inch up, 1.25 inches to the right, and 1.0 inch downrelative to the head centered position of FIG. 6A.

FIG. 8 shows the positioning mechanism for the display surface of theliquid crystal display of FIG. 5.

FIG. 9A is a block diagram of an electrical circuit for developingcontrol signals for the positioning mechanism of FIG. 8.

FIG. 9B shows a graphical representation of an exemplary function ofimage distance versus vehicle speed implemented in the circuit of FIG.9A to determine the axial position of the positioning mechanism of FIG.8.

FIG. 10 is a schematic diagram taken along lines 10--10 in FIG. 1showing the optical components positioned at the output of theprojection lens system to avert display surface damage resulting fromentry of sunlight into the projection lens system of lens system of FIG.5.

FIG. 11 shows the orientation of the optical components depicted in FIG.10.

FIG. 12 is a diagram of the optical components of the projection lenssystem comprising a part of the head-up display system of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 is a diagram of an optical display system 10 installed as ahead-up display system for an automobile 12. Head-up display system 10can be employed in other motor vehicles, such as, for example, trucksand boats. Head-up display system 10 includes a motor vehicle windshield14 that functions as a vision unit having reflective surfaces 16 throughwhich an observer 18 is situated inside the automobile can view anoutside world scene. In the following description, observer 18 is thedriver of the automobile. The present invention can be practiced withwindshields of many design types, such as a trilayer windshield havingtwo glass plies and a polycarbonate interlayer positioned between them,a bilayer windshield having a glass ply and an inner polycarbonatelayer, a single glass ply windshield, an antilacerative windshield, anda combination windshield formed from glass and plastic.

Reflective surfaces 16 reflect source information-carrying light rays inthe form of image symbology appearing forward of automobile 12 in thedriver's instantaneous display field of view 20 (shown in vertical crosssection in FIG. 1 and in horizontal cross section in FIG. 2). (Exemplarydisplay symbology is shown in FIG. 6). Reflective surfaces 16 are thereflection nonenhanced inner and outer surfaces of windshield 14 itself.Windshield 14 may also carry a reflection enhancement material of eitherthe wavelength nonselective type such as an aluminum coating or thewavelength selective type such as a hologram or a multilayereddielectric coating. A reflection enhancement material can be applied ona surface of or embedded within windshield 14. Other suitable reflectionenhancement materials could include an indium tin oxide coating used ina heatable windshield and a light and heat reflective coating used oncertain windshields.

With reference to FIGS. 1 and 5, the source information-carrying lightrays emanate from an information source 22 to a projection lens system24, both of which are mounted beneath the top surface 26 of theautomobile dashboard 28. Information source 22 includes a light source30 and a liquid crystal display 32 separated by a light pipe 34.Information source 22 and projection lens system 24 are mounted behindthe engine compartment firewall 36 but outside the passenger compartment38 to protect liquid crystal display 32 from the heat generated by lightsource 30. The source information-carrying light rays propagate fromliquid crystal display 32 and emerge from output 40 of projection lenssystem 24 through an opening 42 in top surface 26 of dashboard 28 forreflection off reflective surfaces 16 toward driver 18.

Driver 18 views the outside world scene within a normal operationalfield of view 44 (shown in vertical cross section in FIG. 1 and inhorizontal cross section in FIG. 2), which is positioned above and issubstantially nonoverlapping of display field of view 20. Display fieldof view 20 describes a maximum angle of about 12° in the vertical planebetween lines 46 and 48, line 46 describing an angle 50 of about 5°relative to the horizon line 52. Operational field of view 44 describesan angle of about 15° in the vertical plane between lines 54 and 46,line 54 describing an angle 56 of about 10° relative to horizon line 52.Each of the lines 46, 48, 52, and 54 is drawn from point 58, which isthe center of a vehicle eye ellipse 59. The eye ellipse 59 for a vehicleis that nominal spatial envelope for safe operation of the vehicle.These lines intersect windshield 14 to define the vertical dimensions ofthe fields of view 20 and 44 on windshield 14, as described in greaterdetail below.

FIG. 2 is a plan view of windshield 14 showing the position ofwindshield 14 used for the driver's operational field of view 44 of theoutside world scene and the position of windshield 14 through whichdriver 18 views the total display field of view 20 of the sourceinformation. With reference to Figs. 1 and 2, operational field of view44 defines the normal region of space driver 18 views the real worldfrom the head centered position while operating automobile 12. The headcentered position is the orientation of the driver's head when both eyesare symmetrically positioned about point 58. Display field of view 20defines an area on windshield 14 through which driver 18 views thedisplay. The top side margin of display field of view 20 is angularlydisplaced from the driver's direct line of sight by angle 50. FIG. 2shows the display image symbology 60 (rotated into the plane of thefigure for purposes of clarity) appearing at a distance forward ofdriver 18 and beyond reflective surfaces 16. The intersection of displayfield of view 20 with windshield 14 has a vertical dimension 120 and ahorizontal dimension 122. The intersection of operational field of view44 with windshield 14 has a vertical dimension 124 and a horizontaldimension 126.

FIGS. 3A and 3B are fragmentary cross sectional side views showing thereflective surfaces 16 of, respectively, a reflection nonenhanced andreflection enhanced windshield 14. With reference to FIG. 3A, awindshield 14 made of glass has an inner major surface 62 and an outermajor surface 64 that form air-glass boundaries. An exemplary light ray66 propagating from projection lens system 24 (FIG. 1) strikes innersurface 62 and decomposes into a transmission component 68_(t) ofgreater intensity and a reflection component 68_(r) of lesser intensity.Transmission component 68_(t) propagates through windshield 14 andstrikes the air-glass interface formed by outer surface 64. Lightcomponent 68_(t) decomposes into a transmission component 70_(r) ofgreater intensity and a reflection component 70_(r) of lesser intensity,the transmission component propagating through the air and unnoticed bydriver 18, and the reflection component 70_(r) propagating back throughwindshield 14 toward the air-glass interface of inner surface 62.

Reflection component 70_(r) strikes inner surface 62 and decomposes intoa reflection component 72_(r) of lesser intensity that propagates backthrough windshield 14 and a transmission component 72_(t) of greaterintensity that propagates toward driver 18 after refraction at innersurface 62. The intensity of reflection component 72_(r) is ofinsignificant amount. The total intensity "R" representing the sum ofreflection component 68_(r) and transmission component 72_(t) is of asufficient amount to provide driver 18 with source information of highbrightness.

With reference to FIG. 3B, windshield 14 includes a reflectionenhancement element 78, which can be, for example, a wavelengthselective element such as a hologram or a wavelength nonselectiveelement such as a metallic coating. Reflection enhancement element 78intercepts light component 68_(t) and reflects a substantial portion ofit as light component 68_(r78). The remaining light energy intransmission component 68_(t) strikes outer surface 64, propagates backthrough windshield 14, and strikes reflection enhancement element 78.The reflection component 70_(r) propagating through reflectionenhancement element 78 of FIG. 3B is of substantially less intensitythan that of reflection component 70, propagating only through the glassof FIG. 3A. The total intensity "R" representing the sum of reflectioncomponents 68_(r) and 68_(r78) and transmission component 72_(t) isabout 50% greater than the total intensity "R" of the windshield of FIG.3A. It will be appreciated that the intensities of unused lightcomponents 70_(t) and 72_(r) of FIG. 3B are substantially less thanthose of FIG. 3A.

FIG. 4 is a graph showing the reflection characteristics of a singleglass surface as a function of the angle of incidence Θ_(i). In FIG. 4,the single surface corresponds to inner surface 62 of FIGS. 3A and 3B;and the curves R.sub.⊥ and R.sub.∥ represent the electric fieldcomponents perpendicular and parallel, respectively, to the lightwavefront plane of incidence, which has an angle of incidence Θ_(i)relative to a local surface normal 76.

FIG. 4 reveals that for the reflection nonenhanced windshield 14 of FIG.3A having a refractive index n_(t) =1.5 and an angle of incidence Θ_(i)of about 68°, the average reflectance of a single surface is about 16%of incident light and the average reflectance of two surfaces is about24%. This represents the superposition of 16% and 8% contributions oflight components 68_(r) and 72_(t), respectively. FIG. 4 reveals thatthe reflection enhanced windshield of FIG. 3B under the same conditionsreflects about 33.4% of incident light. This represents thesuperposition of 16%, 15%, and 2.4% contributions of light components68_(r), 68_(r78), and 72_(t), respectively. Angles of incidence Θ_(i) ofsuch magnitudes are readily available with windshields inclined atrelatively large slopes. FIG. 4 indicates that an angle of incidenceΘ_(i) of about 60° provides for two surfaces a reflectance of about 18%,which is about the minimum reflectance needed to provide acceptabledisplay brightness.

FIG. 5 is a top view of automobile 12 with a portion of dashboard 28 cutaway to show the mounting of information source 22 and projection lenssystem 24. With reference to FIG. 5, a preferred information source 22includes a broad band multispectral light source 30 and a transmissiveliquid crystal display 32 but can be, for example, a cathode-ray rube ora light emitting diode display. Light source 30 and liquid crystaldisplay 32 are separated by an internally reflecting tubular light pipe34. Light source 30 is preferably a high intensity projection lamp, suchas, for example, a type EYF 12 volt quartz lamp manufactured by GeneralElectric Co. This lamp dissipates about 75 watts of heat at its maximumbrightness. To increase the display contrast, light source 30 preferablyincludes a faceted dichroic reflector that directs the light of about11,500 center candlepower with a 14° beam spread along the length oflight pipe 34 but allows the dissipation of heat in all directions.Liquid crystal display 32 comprises a liquid crystal cell 80 (FIG. 8) ofthe twisted nematic type configured as a matrix of transmissive segmentsthat cooperate with light polarizing filters to selectively transmitlight to synthesize source information in the form of symbology 60. Thesegments of liquid crystal cell 80 can be individually addressed for ahigh contrast display. Liquid crystal cell 80 can also be fabricated asan array of pixels to provide, for example a map display. Displaysurface 82 (FIG. 8) is inclined at an angle 84 of about 12° relative tothe display system optic axis 100 for reasons described below withreference to FIG. 5.

Light pipe 34 facilitates the placement of light source 30 at aconvenient location where heat generated by the light source can bedissipated away from liquid crystal display 32. Light pipe 34 could beconstructed of a transparent plastic tube or rod having a reflectivecoating, but is preferably constructed of aluminum tubing that isdesigned to efficiently transfer light emitted from light source 30 toliquid crystal display 32. The inner surface of light pipe 34 ispreferably polished, nickel plated, or extruded such that the surfacefinish is smooth and shiny, thereby maximizing light transmission. Theuse of aluminum promotes the efficient dissipation of heat generated bylight source 30.

Projection lens system 24 projects the source information appearing ondisplay surface 82 of liquid crystal display 32 so that the sourceinformation reflected by windshield 14 appears to come from a distanceof between about two and ten meters forward of the vehicle. Projectionlens system 24 includes multiple lens elements fabricated from eitherplastic or glass materials and has optical light directing propertiesthat project source information forward of automobile 12 and compensatefor optical aberrations resulting from the complex shape of windshield14. The optical design of projection lens system 24 is further describedbelow.

Projection lens system 24 is composed of two sections joined at theirends to form a structure having a right angle bend, one section 102extending parallel to the bottom side margin 104 of windshield 14 andthe other section 106 being oriented so that it directs light carryingthe source information up toward reflective surfaces 16 of windshield14. An internally mounted mirror folds the optic axis 100 of projectionlens system 24. The right angle bend allows projection lens system 24 toclear the steering column, but a suitable lens system could be designedwith an unfolded optic axis.

The use of wavelength selective coating in conjunction with thin filmcolor transmission filters applied to the light incident surface 108(FIG. 8) of liquid crystal cell 80 can be used to equalize thereflectance of different colors of light carrying the sourceinformation. This can be achieved using a windshield having reflectivesurfaces of the reflection nonenhanced and wavelength selectivereflection enhanced types.

For example, the preferred light source 30 employed in the presentinvention emits spectral components of red and green light. Theintensity of the red light is greater than that of the green light.Providing a hologram 78 constructed to reflect the green light and usingthe inner and outer surfaces to reflect most of the red light provides amulticolor display image in color components of perceptibly equalintensities.

FIG. 6 shows the instantaneous display field of view 20 from the headcentered position and the total display field of view 112 of head-updisplay system 10. With reference to FIG. 6, instantaneous display fieldof view 20 is that portion of the total system display field of view 112visible from a fixed head position. Each of the driver's eyes sees adifferent portion of instantaneous display field of view 20, the righteye seeing portion 114 and the left eye seeing a portion 116. Theinstantaneous display field of view 20 is defined by the size of theexit aperture of projection lens system 24 and its position relative toreflective surfaces 16 of windshield 14, and the distance betweenwindshield 14 and driver 18. Total display field of view 112 is definedby the design of projection lens system 24. A different portion of totaldisplay field of view 112 is visible instantaneously to driver 18 fromdifferent vertical head positions. Liquid crystal display 32 isadjustable so that display symbology 60 can be centered within thevertical limits of vehicle eye ellipse 59 (FIGS. 1 and 2).

FIG. 6 shows that the design of head-up display system 10 makes visibleall of the source information from the head centered position. Head-updisplay system 10 provides an instantaneous display field of view 20from any head position within an eye ellipse 59. Display field of view20 has a vertical angular subtense 117 of about 6.75° and a horizontalangular subtense 119 of about 10.75°. The dimensions of symbology 60 arescaled so that all of the source information can be visible from anyfixed head position within the limits of eye ellipse 59, as describedbelow in greater detail. The total display field of view 112 with headmotion has a vertical angular subtense 128 of about 12° and a horizontalangular subtense 129 of about 14.75°.

FIGS. 7A-7D show the instantaneous display fields of view from a varietyof head positions. For purposes of clarity, the area occupied bysymbology 60 has been shown in broken outline and FIG. 6 has beenredrawn to provide the head center position as a reference against whichinstantaneous fields of view positions of FIG. 7A-7D can be compared.FIGS. 7A and 7C show that all of the display is visible to the observerwith about 7.5 cm of horizontal head motion. FIGS. 7B and 7D show thatportions of the source information could be lost to the observer ifdisplay surface 96 of liquid crystal display 32 were not adjustable.Horizontal adjustments of display surface 82 permit the display ofsource information to be centered from any vertical head position withinthe eye ellipse. Operators of many different heights can, therefore,adjust the display for easy viewing.

FIG. 8 shows a positioning mechanism 130 for positioning display surface82 of liquid crystal display 32 in the horizontal and axial directions.With reference to FIG. 8, positioning mechanism 130 comprises a pair ofmotor-driven micrometers 132 and 134 of similar design that arecomponents of a horizontal translation stage 136 and an axialtranslation stage 138, respectively. Horizontal translation stage 136includes a support bracket 140 that is secured to a plate 142 positionedacross the input end 144 of projection lens system 24. Plate 142 hasthree bushings 146 (only two shown) through which parallel rods 148 offixed length pass. Rods 148 are positioned at and extend outwardly ofinput end 144 of projection lens 24 so that plate 142 can slidably movein the direction of optic axis 100, as will be described below.

Plate 142 has a rectangular aperture 150 across which liquid crystaldisplay 32 is slidably mounted for horizontal movement along a pair ofparallel U-shaped guide members 152 secured to plate 142. Micrometer 132includes a tubular body 154 that houses an extensible rod 156 whose freeend is secured to liquid crystal display 32. Extensible rod 156 moves ineither direction along the length of tubular body 154 in response to afirst electrical control signal, which can be manually applied by driver18 situated in automobile 12. Moving extensible rod 156 changes theposition of display surface 82 of liquid crystal display 32 in adirection transverse of optic axis 100. This has the effect of changingthe vertical position of the source information display to driver 18.The reason that horizontal movement of liquid crystal display 32 changesthe vertical position of the source information display is thatprojection lens system 24 is configured to fold optic axis 100 andthereby change the propagation direction of light passing through it.The ability to change the vertical position of the source informationfacilitates viewing by drivers of different heights.

Axial translation stage 138 includes a support bracket 158 that issecured to the body of automobile 12. Micrometer 134 includes a tubularbody 160 that houses an extensible rod 162 whose free end is secured toplate 142 and which moves in a direction generally parallel to opticaxis 100 in response to a second electrical control signal. Movingextensible rod 162 changes the distance between display surface 82 andinput end 144 of projection lens system 24. This has the effect ofchanging the distance between the image of the display information 60and driver 18. The ability to change this distance promotes safeoperation of automobile 12 by allowing driver 18 to focus his eyes onthe displayed image at a distance to which driver 18 would be normallydirecting his attention during high speed operation of automobile 12.This distance would normally be the distance between automobile 12 andthe vehicle in front of it. Driver 18 would not, therefore, need to takethe time to refocus his eyes under conditions in which driver 18 hasless time to react to change in driving conditions and situations.

FIG. 9A is a block diagram of an electrical circuit 164 for developingthe first and second electrical control signals delivered to positioningmechanism 130 to adjust the position of liquid crystal display 32. Withreference to FIG. 9A, driver 18 manually produces the first controlsignal by operating a two-pole double-throw switch 166 to position thedisplay information vertically. The poles of switch 166 are connected todifferent ones of voltages of magnitudes +V and -V. Driver 18selectively changes the position of a movable contact 168 of switch 166to make electrical contact with one of the poles and thereby selectivelyprovide either +V or -V to the input conductor of a drive amplifier 170.Drive amplifier 170 provides on its output conductor 172 an electricalcurrent of sufficient magnitude to drive motor-driven micrometer 132.The polarity of the voltage applied to drive amplifier 170 determinesthe direction of drive current flow, thereby controlling the directionof movement of extensible rod 156 and the vertical position of thesource information 60 displayed to driver 18. To maintain the displayinformation in the desired vertical position, driver 18 removes his handfrom switch 166 so that no voltage is applied to drive amplifier 170 tokeep extensible rod 156 of micrometer 132 stationary.

The image distance is automatically derived from the speed of automobile12. A speedometer 174 develops a variable frequency output signal thatis delivered on a conductor 176 to the input of a frequency-to-digitalconverter 178. A parallel digital word representing the speed ofautomobile 12 appears on the outputs of analog-to-digital converter 178and is delivered on conductors 180 to the address inputs of a read onlymemory 182. Read only memory 182 is a look-up table preprogrammed toinclude digital words representing image distances to which differentvehicle speeds correspond. The appropriate digital word is delivered ona conductor 184 to the input of a digital-to-analog converter 186, whichconverts the digital word to an analog signal voltage. A scaler-driveamplifier 188 receives on a conductor 190 the analog signal voltagepresent on the output of digital-to-analog converter 186 and conditionsit to deliver on a conductor 192 a signal of the proper magnitude andpolarity to move extensible rod 162 of micrometer 134 to the properposition. To keep the image distance fixed, no voltage signal is appliedto micrometer 134 until the vehicle speed changes.

FIG. 9B shows an exemplary curve 194 from which the digital wordsinscribed in read only memory 182 can be derived. Curve 194 shows thatthe image distance from driver 18 remains constant at about 6 meters forvehicle speeds of less than about 50 km/hr. The image distance increasesasymptotically toward infinity as the vehicle speed increases to about100 km/hr. Of course, other image distance profiles as a function ofvehicle speed can be inscribed into read only memory 182.

FIG. 10 is a schematic diagram of the optical components that cooperatewith projection lens system 24 and liquid crystal display 32 to presentthe source information to driver 18 and to prevent damage to displaysurface 82 resulting from the entry of sunlight into the output end 40of projection lens system 24. Projection lens system 24 is rotated tothe plane of FIG. 10 to show the optical components positioned adjacentoutput end 40. With reference to FIG. 10, sunlight of sufficientintensity entering output end 40 of projection lens system 24 convergesto and can cause melting of a small region of display surface 82 ofliquid crystal cell 80. Display surface 82 can withstand a burnthreshold density of about 500 W/in², but the focal power of a preferredprojection lens system 24 can provide a sunlight power density of asmuch as about 2500 W/in² at the region.

It is known that imparting circular polarization to sunlight enteringthe output end of a lens system can reduce the intensity of the sunlightas it propagates through the lens system. Moreover, a circular polarizerreduces the intensity of sunlight reflected from the surfaces of lenselements 258, 260, 262, and 264 and visible to driver 18. A circularpolarizer 202 that comprises a neutral density linear polarizing filter204 and a quarter-wave plate 206 positioned as shown in FIG. 10accomplish this task for projection lens system 24. Further rejection ofincoming sunlight is accomplished by the inclusion of an infrared lightrejection filter 208, which eliminates the infrared spectral componentsof incident light and thereby reduces the quantity of sunlight enteringprojection lens system 24. Rejection filter 208 is most effective in theposition immediately adjacent output end 40 of projection lens system24.

A quarter-wave plate 210 is positioned to receive but has no materialeffect on the randomly polarized incident sunlight as it strikespolarizing filter 204. The function of quarter-wave plate 210 isdescribed below with reference to FIG. 11. A neutral density linearpolarizing filter 212 positioned to receive the light propagating fromlight source 30 (FIG. 5) and exiting light pipe 34 linearly polarizesthe light that illuminates liquid crystal display 32.

Liquid crystal display 32, which includes liquid crystal cell 80preferably of the twisted nematic type, changes the polarizationdirection of the light passing through it in response to a voltagesignal applied to each one of the display segments. Polarizing filter204 functions as an analyzer for liquid crystal display 32. Aquarter-wave plate 214 positioned between input end 144 of projectionlens system 24 and liquid crystal display 32 imparts circularpolarization to light exiting liquid crystal display 32 for the reasonsstate below with reference to FIG. 11.

FIG. 11 is a diagram showing the arrangement and orientations of theoptical components depicted in FIG. 10. (For purposes of clarity,projection lens system 24 is represented in FIG. 11 as a single lenselement.) The optical components are selected and arranged so thatgreatly attenuated circularly polarized sunlight strikes display surface82 of liquid crystal display 32 and source information carried bycircularly polarized light of maximum intensity exits quarter-wave plate210 for display to driver 18.

With reference to FIGS. 10 and 11, randomly polarized sunlightpropagating from left to right in FIG. 11 along optic axis 100 passesthrough quarter-wave plate 210 and strikes polarizing filter 204. Theabsorption axis 216 of polarizing filter 204 absorbs about 50% of therandomly polarized incident sunlight energy. Transmission axis 218 ofpolarizing filter 204 transmits horizontally polarized sunlight toquarter-wave plate 206, whose optic axis 220 is inclined at a 45° anglerelative to polarization axes 216 and 218 of polarizing filter 204 totransmit left-circularly polarized sunlight to rejection filter 208.Circularly polarized sunlight emerges from quarter-wave plate 206 andrejection filter 208 and enters output end 40 of projection lens system24. The intensity of the sunlight entering projection lens system 24 issubstantially reduced by reason of the insertion losses of infraredrejection filter 208, polarizing filter 204, and quarter-wave plates 206and 210, together with the operation of rejection filter 208 andabsorption axis 218 of polarizing filter 204. The resulting reduction insunlight intensity in conjunction with the introduction of circularpolarization reduces the sunlight power density on display surface 82 ofliquid crystal display 32 below the 500 W/in.² burn threshold.

The randomly polarized light emanating from light source 30 (FIG. 5) andpropagating from right to left in FIG. 11 along optic axis 100 strikespolarizing filter 212. The absorption axis 222 of polarizing filter 212absorbs about 50% of the randomly polarized incident light energy.Transmission axis 224 of polarizing filter 212 transmits horizontallypolarized light to liquid crystal display 32, whose optic axis 226 isinclined at an angle with respect to polarization axes 222 and 224 ofpolarizing filter 212 to transmit either vertically or horizontallypolarized light in response to the voltage signals applied to thesegments of liquid crystal display 32. The light exiting display surface82 of liquid crystal display 32 strikes quarter-wave plate 214, whoseoptic axis 228 is inclined at a 45° angle relative to the polarizationdirections of light exiting liquid crystal display 32 to providecircularly polarized light to input end 144 of projection lens system24.

Since optic axes 220 and 228 of the respective quarter-wave plates 206and 214 are orthogonally aligned, there is zero net retardation impartedto them to light exiting display surface 82 of liquid crystal display32. Only horizontally polarized light propagates through polarizationaxis 218 of polarizing filter 204, which functions as an analyzer, aswas stated above. The optic axis 230 of quarter-wave plate 210 isinclined at a 45° angle relative to polarization axes 216 and 218 ofpolarizing filter 204 to provide left-circularly polarized light toreflective surfaces 16 of windshield 14 (FIG. 1). The circularlypolarized light ensures that driver 18 wearing polarized sunglasses willsee the display source information.

It will be appreciated that the use of quarter-wave plates 206 and 214promote the maximum transmissivity of light propagating from liquidcrystal display 32 because polarizing filter 204 either completelyabsorbs or completely transmits the linearly polarized light incident toit. Moreover, since circularly polarized light propagates through lenssystem 24, the set of optical components 204, 206, and 210 and the setof the optical components 80, 212, and 214 can be angularly displacedrelative to each other (i.e., rotated) about optic axis 100 withoutchanging the transmissivity of either the sunlight or the light carryingthe source information.

Antireflection coatings are applied to all of the exposed surfaces ofthe lens elements of projection lens system 24 to maximize lighttransmission through it. The use of wavelength selective reflectionenhancement material in windshield 14 would also reduce the amount ofsunlight entering projection lens system 24.

FIG. 12 is a diagram of the optical components of projection lens system24. With reference to FIG. 12, projection lens system 24 projects avirtual image of display information into the driver's field of view byreflecting off reflective surfaces 16 the light emanating from displaysurface 82. Reflective surfaces 16 of windshield 14 are curved with anasymmetrical shape in the portion of windshield 14 intended for use;therefore, projection lens system 24 is designed to compensate fornonsymmetric image aberrations caused by the shape of windshield 14. Aspecial aspheric surface is used in the design of projection lens system24 to correct the nonsymmetric aberrations and distortion introduced.The preferred projection lens system 24 may be designed to employ eitherglass or plastic elements.

The design uses four elements, three of which are of conventionalrotationally symmetric types with spherical surfaces, and oneplano-convex element in which the convex surface is of special asphericshape. FIG. 12 shows the general layout of projection lens system 24 intwo lens "doublet" groups and an internal fold mirror. The first elementpositioned nearest the windshield is rotated about optic axis 100, andits cross sectional projection is shown in FIG. 12.

The spectral range for projection lens system 24 includes visible lightranging from between 490 and 620 nanometers. This range of wavelengthsrequires that color correction be controlled and

checked. The instantaneous field of view is determined by the distanceaway and size of the exit aperture with respect to the design eye, i.e.,the center of ellipse 59, of head-up display system 10. Fixing the aboveparameters with respect to each other establishes the maximuminstantaneous field of view. The total field of view depends on theallowed excursion of the driver's head, the size of display surface 82,and the focal length of projection lens system 24. The image presentedto driver 18 is preferably not positioned at infinity.

The design of a lens system that compensates for the curvature ofwindshield 14 requires that the windshield curvature itself becharacterized. This is accomplished by obtaining windshield sag dataeither by empirical measurements or from the windshield manufacturer'sdesign specifications. Table I presents windshield sag data for anexemplary windshield. The windshield sag data was measured relative to abest fit plane and the compound angle of the windshield relative to anorthogonal coordinate system centered at eye ellipse 59 of head-updisplay 10. The angle of the best fit plane is characterized by ahorizontal tilt angle equal to 2.712° and a vertical tilt angle equal to68.325°. The X and Y coordinates are measured with respect to the centerof region 20 on windshield 14.

                  TABLE I                                                         ______________________________________                                        WINDSHIELD SAG DATA                                                                                        Measured                                                                              Computer                                 Position                                                                             X Coordinate                                                                             Y Coordinate                                                                             Sag     Sag                                      ______________________________________                                         1     2.9881     2.9955     0.1235  0.1235                                    2     3.0002     1.4804     0.1180  0.1184                                    3     3.0045     -0.0172    0.1121  0.1122                                    4     3.0030     -1.5071    0.1048  0.1041                                    5     3.0070     -3.0157    0.0931  0.0931                                    6     1.5060     2.9982     0.1495  0.1497                                    7     1.49838    1.4963     0.1452  0.1451                                    8     1.4958     -0.0133    0.1392  0.1393                                    9     1.5079     -1.5032    0.1316  0.1315                                   10     1.4977     -3.0085    0.1215  0.1214                                   11     -0.0053    2.9991     0.1597  0.1598                                   12     -0.0086    1.4949     0.1552  0.1551                                   13     0.0        0.0        0.1493  0.1493                                   14     -0.0079    -1.1508    0.1419  0.1418                                   15     -0.0043    -3.0129    0.1318  0.1319                                   16     -1.5044    2.9952     0.1517  0.1513                                   17     -1.5016    1.4917     0.1467  0.1462                                   18     -1.5041    -0.0122    0.1404  0.1403                                   19     -1.5021    -1.5091    0.1330  0.1329                                   20     -1.4947    -3.0199    0.1230  0.1231                                   21     -3.0045    2.9938     0.1224  0.1224                                   22     -3.0079    1.4933     0.1169  0.1166                                   23     -3.0034    -0.0140    0.1103  0.1104                                   24     -2.9927    -1.5163    0.1029  0.1030                                   25     -2.9886    -3.0098    0.0934  0.0934                                   ______________________________________                                    

The data presented in Table I were fit to the following two dimensionalthird order rotationally nonsymmetric polynomial:

    Sag=A+BX+CY+EX.sup.2 +FXY+GY.sup.2 +HX.sup.3 +IX.sup.2 Y+JXY.sup.2 +KY.sup.3,

where A=1.49321×10⁻¹, B=5.64422×10⁻⁴, C=4,32024×10⁻³, E=-4.20469×10⁻³,F=1.27263×10⁻⁵, G=-3,81329×10⁻⁴, H=9.62663×10⁻⁵, I=3.35038×10⁻⁵,j=-2.46865×10⁻⁵, K=3.48652×10⁻⁵. This polynomial form constitutes thecomplex Windshield curvature definition used by the optical designer andclosely matches the measured data, as indicated in Table I. The aboveequation represents "Asphere (1)" in Table II below.

Projection lens system 24 compensates for aberrations arising in head-updisplay 10. Projection lens system 24 includes four lens elementsarranged in a first "field flattener" group 250 of two elementspositioned near liquid crystal display 32 and a second or "exit" group252 of two elements positioned near exit aperture 254 at output end 40.Projection lens system 24 is folded inboard clear of the steering column(not shown) of automobile 12. An internal fold mirror 256 folds opticaxis 100 in the region between the two groups of lens elements.

The field flattener group 250 includes a strong positive lens element258 cemented to a negative element 260. The exit group 252 includes awedged rotationally nonsymmetric anamorphic asphere 262 and a negativemeniscus lens element 264 for color correction. Lens exit aperture 254(FIG. 10), which determines the field of view characteristics of head-updisplay 10 is 5 in. in diameter truncated to a 4 in. by 4 in. square,which provides a field of view of rectangular format. The use of lens"doublets" provides color correction. The use of asphere 262 inprojection lens 24 introduces asymmetry into projection lens system 24that causes its focal plane to tilt. The tilt angle 84 of about 12° fordisplay surface 82 on which the source information image appearscorrects for the asymmetry in projection lens system 24.

Projection lens system 24 is adaptable for use with a great number ofwindshields of vehicles of different types and body styles ofsubstituting into exit group 252 an aspheric element 262 characterizedby the proper windshield sag properties and adjusting the tilt angle 84of display surface 82. The remaining lens elements 258, 260, and 264 andfold mirror 256 can be kept constant. This can be done because asphere262 compensates for the nonsymmetric or "aspheric" aberrationsintroduced by the particular windshield, and lens elements 258, 260, and264 correct for spherical and skew ray astigmatism and coma aberrationsintroduced by the optical system geometry of head-up display 10.

Tables II and III set forth the fabrication data for projection lenssystem 24.

With reference to Tables II and III, the surfaces a-p correspond ingeneral to lettered surfaces in FIG. 12, where "a" corresponds to theimage driver 18 sees and surface "p" corresponds to display surface 84of liquid crystal display 32. In each instance, the radius of thesurface is given and the shape of each surface is spherical except forsurfaces a, h, and p which are flat, and surfaces d, e, f, and g whichare aspheric and relate to windshield 14 and asphere 262.

A positive radius indicates the center of curvature is to the right ordown in FIG. 12, and a negative radius indicates the center of curvatureis to the left or up in FIG. 2. Dimensions are given in inches, and thethickness is the axial distance to the next surface.

                  TABLE II                                                        ______________________________________                                        LENS FABRICATION DATA                                                                   Surface                                                                       Description                                                                              Thickness or                                             Surface   Radius     Separation                                               ______________________________________                                        a   (image)   infinite   -120.0  (distance from                                                                image to design eye)                         b   (design              17.5    (distance from                                   eye)                                                                                                       design eye to pupil)                         c   (pupil)              11.5    (distance from pupil                                                          to windshield)                                             Decenter (1)                                                    d   Aspheric  Asphere (1)                                                                              0.09                                                     windshield                                                                e             Asphere (1)                                                                              -0.09                                                f             Ashpere (1)                                                                              -5.6                                                               Reverse                                                                       Decenter (2)                                                                  Decenter (3)                                                                             -0.0290                                                            Decenter (4)                                                    g   Asphere (2)                                                                             -1.1084                                                                       Return (4)                                                                    Decenter (5)                                                    h             infinite   -0.0441                                                            Return (5)                                                      i             -4.3650    -0.350                                               j             -3.0260    -2.450  (distance from                                                                surface j to optic                                                            axis fold point)                             k   (optic axis          -2.160  (distance from                                   fold point)                  optic axis fold                                                               point to surface j)                          l             -2.5590    -2.2767                                              m              6.1400    0.00                                                 n              6.140     -0.750                                               o             -3.497     -1.2414                                                            Decenter (6)                                                    p   (image)   infinite                                                        ______________________________________                                    

                                      TABLE III                                   __________________________________________________________________________    DECEN-                                                                        TER  X    Y     Z    ALPHA BETA  GAMMA                                        __________________________________________________________________________    D (1)                                                                              0.0000                                                                             0.0000                                                                              0.0000                                                                             68.3250                                                                             -2.7120                                                                             0.0000                                       D (2)                                                                              0.0000                                                                             0.0000                                                                              0.0000                                                                             -68.3250                                                                            2.7120                                                                              0.0000                                       D (3)                                                                              0.0000                                                                             -0.0507                                                                             0.0000                                                                             0.0000                                                                              0.0000                                                                              0.0000                                       D (4)                                                                              0.1254                                                                             -1.7579                                                                             0.0000                                                                             -11.0442                                                                            -0.8880                                                                             -48.5404                                     D (5)                                                                              0.0000                                                                             0.0000                                                                              0.0000                                                                             6.0793                                                                              0.0000                                                                              -39.2098                                     D (6)                                                                              0.0000                                                                             0.0878                                                                              0.0000                                                                             -11.9001                                                                            -0.8916                                                                             0.0000                                       DECENTERING CONSTANT KEY:                                                     TYPE           ORDER OF APPLICATION                                           DECENTER       DISPLACE (X, Y, Z)                                                            TILT (ALPHA, BETA, GAMMA)                                                     REFRACT AT SURFACE                                                            THICKNESS TO NEXT SURFACE                                      DECENTER AND RETURN                                                                          DECENTER (X, Y, ALPHA, BETA, GAMMA)                                           REFRACT AT SURFACE                                                            RETURN (-GAMMA, -BETA, -ALPHA, -Y, -X)                                        THICKNESS NEXT TO SURFACE                                      REVERSE DECENTER                                                                             REFRACT AT SURFACE                                                            TILT (-GAMMA, -BETA, -ALPHA)                                                  DISPLACE (-Y, X)                                                              THICKNESS TO NEXT SURFACE                                      __________________________________________________________________________

Aspheric surfaces d, e, and f are described by the three dimensionalthird order or polynomials set forth above for the sag, and the asphericsurface g for asphere 262 is characterized by the following expression:##EQU1## where CY=-0.19363896, KY=0.018526, CX=-0.19652289,KX=-0.017610, AR=4.90202×10⁻⁷, BR=-7.65670×10⁻⁶, CR=2.9547×10⁻⁷,DR=4.58690×10⁻⁹, AP=-12.1100, BP=0.115192, CP=0.0738795, DP=0.0471630.The above equation represents "Asphere (2)" in Table II.

A decenter defines a new coordinate system (displaced and/or rotated) inwhich subsequent surfaces are defined. Surfaces in Table II following adecenter are aligned on the local mechanical axis (Z-axis) of the newcoordinate system. The new mechanical axis remains in use until changedby another decenter. The order in which displacements and tilts areapplied on a given surface is specified using different decenter typesand generate different new coordinate systems. Decenters used in thedesign III. The parameters α, β, and γ, are in degrees.

It will be obvious to those having skill in the art that many changesmay be made in the above-described details of the preferred embodimentof the present invention without departing form the underlyingprinciples thereof. For example, additional reflection enhancement maybe achieved by using multiple holograms that reflect differentwavelengths of light. The scope of the present invention should bedetermined, therefore, only by the following claims.

We claim:
 1. A head-up display system for a vehicle, comprising:a visionunit including a vehicle windshield with a reflective surface throughwhich an observer situated inside the vehicle can view an outside worldscene and can view an instantaneous display field of view postponedwithin a total display field of view; information source means forproviding source information carried by light propagating along a fixedoptical axis for reflection by the reflection surface of the vision unitfor display within the total display field of view; and positioningmeans operatively connected to the information source means forselectively positioning the information source means at differentlocations relative to the optical axis in a direction transverse to theoptical axis so that the source information is positioned within theinstantaneous display field of view for display to the observer.
 2. Thesystem of claim 1 which further comprises a projection lens systempositioned along the optical path between the reflective surface of thevision unit and the information source means and having opticallight-directing properties for compensating for aberrations caused bythe reflective surface of the vision unit so that the light reflected byit is of substantially aberration-free character for viewing by anobserver.
 3. The system of claim 2 in which the vision unit has aprimary operational field of view within which the observer views theoutside world scene and in which the total display field of view isgenerally located outside the primary operational field of view.
 4. Thesystem of claim 1 in which the vehicle includes a dashboard having a topsurface beneath which the information source means is mounted and inwhich the vision unit comprises a windshield having a major surfacebounded by a lower side margin, the top surface of the dashboard beingpositioned proximal to the lower side margin of the windshield andhaving an opening through which light carrying the source informationpropagates to strike the major surface of the windshield.
 5. The systemof claim 1 which further comprises a projection lens system thatconditions the light carrying the source information so that the sourceinformation reflected by the vision unit appears to come from a finitedistance forward of the vision unit and the observer.
 6. The system ofclaim 5 in which the finite distance ranges from between about two andten meters forward of the observer.
 7. The system of claim 5 in whichthe projection lens system includes a rotationally nonsymmetric lenselement.
 8. The system of claim 7 in which the projection lens systemincludes a plastic lens component.
 9. The system of claim 2 in which theprojection lens system includes a rotationally nonsymmetric lenselement.
 10. The system of claim 9 in which the projection lens includesa plastic lens component.
 11. The system of claim 1 in which thereflective surface of the vision unit has different curvatures indifferent sections lying in mutually orthogonal planes.
 12. A head-updisplay system for a vehicle capable of traveling at each of pluralvehicle speeds, comprising:a vision unit having a reflective surfacethrough which an observer situated inside the vehicle can view anoutside world scene; information source means for providing sourceinformation carried by light propagating along a fixed optical axis forreflection by the reflective surface of the vision unit for display tothe observer; projection lens means positioned along the optical axisbetween the reflective surface and the information source means forcompensating for aberrations caused by the reflective surface of thevision unit so that the light reflected by it is of substantiallyaberration-free character for viewing by the observer; and positioningmeans operatively connected to the information source means forpositioning the information source means in a direction along theoptical axis at plural preselected locations that correspond torespective ones of the plural vehicle speeds so that the projection lensmeans provides to the observer an image of the source information from adistance corresponding to the vehicle speed.
 13. The system of claim 12in which the vision unit has a primary operational field of view withinwhich the observer views the outside world scene and in which theinformation source means and the projection lens system cooperate toreflect the source information off the reflective surface of the visionunit within a region generally located outside the primary operationalfield of view.
 14. The system of claim 12 in which the vehicle includesa dashboard having a top surface beneath which the information sourcemeans is mounted and in which the vision unit comprises a windshieldhaving a major surface bounded by a lower side margin, the top surfaceof the dashboard being positioned proximal to the lower side margin ofthe windshield and having an opening through which light carrying thesource information propagates to strike the major surface of thewindshield.
 15. The system of claim 12 in which the projection lensmeans conditions the light carrying the source information so that thesource information reflected by the vision unit appears to come from afinite distance forward of the vision unit and the observer.
 16. Thesystem of claim 15 in which the finite distance ranges from betweenabout two and ten meters forward of the observer.
 17. The system ofclaim 12 in which the projection lens means includes a rotationallynonsymmetric lens element.
 18. The system of claim 17 in which theprojection lens means includes a plastic lens component.
 19. The systemof claim 12 in which the reflective surface of the vision unit hasdifferent curvatures in different sections lying in mutually orthogonalplanes.